The viability of eukaryotic cells relies upon the tight regulation of the initiation of protein synthesis.[unreadable] There is mounting evidence that this regulation involves important conformational changes within the[unreadable] small (40S) subunit of the ribosome. In order to probe the molecular basis and functional roles of[unreadable] these conformational changes in the 40S subunit, we will examine aspects of translation initiation[unreadable] regulation controlled by viral gene expression. Many viruses such as hepatitis C contain internal[unreadable] ribosome entry sites (IRESs) in their messenger RNAs that circumvent conventional initiation. These[unreadable] IRESs do not require the full complement of translation initiation factors to function. This enables[unreadable] viruses to bypass cellular pathways that inhibit translation upon viral infection. Understanding the[unreadable] molecular mechanisms used by viral IRESs will greatly aid in the development of antiviral strategies to[unreadable] combat devastating human viral diseases. The Cricket Paralysis Virus contains and IRES (CrPV IRES)[unreadable] that requires none of the initiation factors, and even circumvents the need for initiator tRNA.[unreadable] Interestingly, the CrPV IRES functions on all classes of eukaryotic ribosome, including human[unreadable] ribosomes. Elucidating its underlying mechanism of action will therefore reveal general features of[unreadable] eukaryotic ribosomes important for translation initiation.[unreadable] The specific aims of the proposal are the following. 1) Map conformational changes in the 40S subunit[unreadable] induced by the hepatitis C Virus IRES during translation initiation, and 2) map conformational changes[unreadable] in the 40S subunit induced by the Cricket Paralysis Virus IRES during translation initiation.